1. Technical Field
The present disclosure relates to a converter, and more particularly, a three-level converter.
2. Description of Related Art
Frequency converter has wide applications in industrial control, and is particularly important in the control and speed regulation of high-power electrical machines. Therefore, the converter is an important part of the energy system. In the industrial applications, the high power or medium power high voltage electrical machines play more and more important roles, which consume about two-thirds of the gross power generation. Accordingly, it is significant to improve the efficiency of these machines.
The existing ROBICON solution (U.S. Pat. No. 5,625,545) employs a frequency converter in which the high-voltage frequency conversion is achieved by a low voltage semiconductor component. However, the frequency converter according to the ROBICON solution requires a phase-shifting transformer disposed in the front, and hence, the transformer is costly and bulky and associated with more energy loss. Further, the ROBICON solution requires a large number of capacitors, resulting in higher cost and failure rate. In view of the foregoing, the solution is massive in volume, low in efficiency and inferior in reliability. Other high voltage frequency converter products use a high-voltage semiconductor component to achieve the high voltage frequency conversion, yet the high-voltage semiconductor is very expensive and associated with higher energy loss, and hence the cost of the system is high.
To overcome the above-mentioned disadvantages, the direct series connection of low-voltage semiconductor switches, such as the insulated-gate bipolar transistor (IGBT), may be a good approach (in which the semiconductor switches are turn-on or turn-off simultaneously, and equivalently operate for a switch component). Meanwhile, to reduce the cost of the system, it is also desirable to replace the phase-shifting transformer with three-level converter using series connection low-voltage IGBTs. Currently, there are products with the 2-level convertor in which a plurality of IGBTs in series connection are applied. However, the design layout of such products is defective due to the inferior maintainability, large commutation loop and high manufacturing cost.
In view of the foregoing, there exist problems and disadvantages in the current technology and further improvements are required for those ordinarily skilled in the art to solve the above-mentioned problems.